Enzymatic synthesis of ionic responsive lignin nanofibres through surface poly(N-isopropylacrylamide) immobilization

Abstract

Surface modification of electrospun lignin nanofibres with poly(N-isopropylacrylamide) (PNIPAM) was conducted through surface-initiated atom transfer radical polymerization (ATRPase) using various biocatalysts under aqueous conditions. Three biocatalysts were investigated, a catalase from bovine liver (CBL), a peroxidase from horseradish (HRP), and a laccase from Trametes versicolor (LTV). All of the biocatalysts were found to successfully graft PNIPAM polymer brushes from the nanofibre surfaces. PNIPAM brush thickness was dependent on enzyme activity and significantly influenced by the reaction conditions; manipulating the type of enzyme, the concentration of reducing reagent and solution pH resulted in PNIPAM brushes of various molecular weight, thickness, and grafting density. Lignin fibre surface immobilized PNIPAM brushes with a thickness of >100 nm and a corresponding molecular weight of more than 1 × 10⁶ g mol⁻¹ and a polydispersity index of less than 1.30 were obtained using LTV as the catalyst and ascorbic acid as the reducing agent. The effect of reducing reagent on the weight percentage of PNIPAM on the nanofibre surface was determined by measuring the enthalpy (ΔH) of the phase transition, which was found to exhibit a similar trend as that of the measured PNIPAM brush thickness. Finally, the low critical solution temperature (LCST) of PNIPAM immobilized lignin nanofibre was similar to that of pure PNIPAM and decreased with increasing the ionic concentration, which simultaneously changed the modified fibre mats from hydrophilic to hydrophobic.